Porous polyethylene filter membrane, related filter, and method

Abstract

To provide a porous filter membrane exhibiting useful and advantageous performance characteristics for filtering a liquid process fluid, which preferably contains flow characteristics (e.g., flow rate, flow time) combined with effective particle removal characteristics (e.g., holding various particle diameters).SOLUTION: A porous polyethylene membrane contains a first surface, a second surface opposite the first, and a thickness between the first surface and the second surface. The membrane exhibits log 10 flow time (sec.) with respect to an average bubble point (pond / square inch), which is less than log 10 flow time (sec.) with respect to an average bubble point based on an expression: log 10 (flow time)=2.757+0.007105* (average bubble point). The flow time is measured by using a flow time test, and the average bubble point is measured by using an average bubble point test.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The following description relates to a porous polyethylene filter membrane comprising two opposing surfaces, a thickness, and a porous structure between the opposing surfaces, as well as to filter components and filters comprising such porous polyethylene filter membranes, to methods of making the porous polyethylene filter membranes, filter components, and filters by coextrusion techniques, and to methods of using the porous polyethylene filter membranes, filter components, or filters. [Background technology]

[0002] Filter membranes and filter products are indispensable tools in modern industry used to remove undesirable materials from useful fluid streams. Useful fluids that can be processed using filter membranes include water, industrial solvents and process fluids, industrial gases used in manufacturing (e.g., in semiconductor manufacturing), and liquids with medical or pharmaceutical uses. Examples of impurities and contaminants that can be removed from fluids by filter membranes include undesirable particles, microorganisms, volatile organic substances, and undesirable chemical species.

[0003] Many filter membranes are designed to remove undesirable materials from liquids. Filter membranes used to filter liquids on a commercial or industrial scale have pore sizes and porosities effective to allow a useful level of flow of the desired liquid through the filter (which may be measured as flow rate, flux, or "flow time"), meaning a level of flow that efficiently supplies an amount of liquid (volume per time) to a commercial system that uses the liquid, such as equipment ("tools") used in semiconductor or microelectronic device manufacturing. Filter membranes used to process (filter) liquids are referred to as "liquid flow" or "liquid flow" filter membranes, as compared to filter membranes designed to process (remove materials by filtration) gaseous fluids.

[0004] A variety of polymeric materials have been used to fabricate filter membranes, including certain polyolefins, polyhaloolefins, polyesters, polyimides, polyetherimides, polysulfones, and polyamides (e.g., nylons). One common material is polyethylene, including high-molecular-weight polyethylene and the type of polyethylene known as "ultra-high-molecular-weight polyethylene" (UPE). Polyethylene (e.g., UPE) filter membranes are commonly used to filter liquid materials used in "wet etch and clean" (WEC) applications for photolithography and semiconductor processing.

[0005] Many different techniques are known for forming porous filter membranes, which can be either gas flow membranes or liquid flow membranes. Exemplary techniques include melt extrusion (e.g., melt casting) and coagulation coating (phase separation) techniques, among others. Various techniques for forming porous polymer filter membranes can often produce different membrane structures with respect to the size and distribution of pores formed within the membrane. Various techniques produce different pore sizes and membrane structures, and these characteristics are sometimes referred to as porous membrane morphology, which can refer to the characteristics of the porous membrane, including the size, shape, uniformity, and distribution of pores within the membrane.

[0006] Examples of membrane morphologies include uniform (isotropic) morphology and asymmetric (anisotropic) morphology. Membranes with pores of substantially uniform size distributed uniformly throughout the membrane are often referred to as isotropic or "homogeneous." Anisotropic (also known as "asymmetric") membranes can be considered to have a morphology that includes a pore size gradient (non-uniform pore distribution) across the membrane; for example, the membrane can have relatively large pores on one membrane surface and relatively small pores on the other membrane surface, with the pore structure varying along the membrane thickness.

[0007] As feature sizes in semiconductor chips and other microelectronic devices become smaller and smaller, there is an increasing need to reduce contaminants in the liquids used in processing these products. Contaminants that may be present in fluids used in processing microelectronic devices and semiconductor chips ("process fluids") cause defects and reduce process yields. Processes used for devices with smaller features require filters that can remove smaller and smaller sized contaminants from the process fluids. To remove smaller particles, filter membranes can be designed with smaller pore sizes. However, as the pore size of a filter membrane decreases, the flow rate of the fluid through the filter typically decreases due to the narrow flow paths of the smaller pore sizes.

[0008] One way to overcome the reduced liquid flow rate (volume per filter area) through a filter membrane due to smaller filter pore size is to increase the amount (i.e., area) of filter through which liquid can flow. A larger filter area allows a larger total volume of fluid to be processed per time at a lower liquid flow rate per filter area. To accommodate the reduced flow rate per filter membrane area, a larger filter area can be provided by using more individual filters. However, because the filter flow rate per area is lower, adding filters or otherwise increasing the amount (area) of filter membranes used to process a given liquid stream increases overall processing costs. Furthermore, there is limited space available in processing tools to increase the size of the required filtration device, meaning that using larger filters or multiple filters is complex and expensive. Summary of the Invention

[0009] The following description relates to porous filter membranes (e.g., "membranes" for short) that exhibit useful or advantageous performance characteristics for filtering liquid process fluids, preferably including useful flow characteristics (e.g., flow rate, flow time) combined with effective particle removal characteristics (e.g., retention of various particle sizes).

[0010] The described membranes have two opposing sides, each side having a surface and a thickness between the two opposing sides. Each surface is associated with a pore structure that extends from the membrane surface to a depth below the surface. One side, which may be referred to as the "dense side" or "retentive side" of the membrane, has smaller pores, higher retention characteristics, resulting in a relatively low flow rate of liquid (exhibiting higher resistance to flow) through the filter. The other side, which may be referred to as the "open side" of the membrane, has larger pores, lower retention characteristics, resulting in a relatively high flow rate of liquid (exhibiting lower resistance to flow) through the filter.

[0011] The dense face has a thickness that is less than the thickness of the open face, where thickness refers to the amount of polymer (by weight) that makes up the dense face compared to the amount of polymer that makes up the open face.

[0012] The described membranes can be made by a coextrusion process, which can be performed with selected and controlled characteristics, such as the relative flow rates of the polymer solutions to produce a dense side with a smaller thickness compared to the open side, and a higher polymer concentration in the heated polymer solution used to form the dense side compared to the open side (to form smaller pores in the dense side), to produce a membrane with the described dense and open sides with the described flow characteristics (e.g., bubble point, flow time).

[0013] The present disclosure may be more fully understood in consideration of the following description of various exemplary embodiments in conjunction with the accompanying drawings, in which: [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 shows a side cutaway view of the membrane described. [Figure 2A] FIG. 2A shows an example of the coextrusion process described. [Figure 2B] FIG. 2B shows an example of the coextrusion process described. [Figure 3] FIG. 3 shows an example of the described filter product. [Figure 4] FIG. 4 shows a plot of log 10 flow time on the Y-axis and mean bubble point on the X-axis for the membranes tested in the examples. DETAILED DESCRIPTION OF THE INVENTION

[0015] Described herein are porous polyethylene filter membranes useful for filtering (removing contaminants from) liquid fluids. The membranes exhibit useful flow characteristics (e.g., flow rate, flow time) for liquids passing through the membrane, combined with useful particle removal properties (e.g., retention of particles of various sizes) to provide efficient filtration performance of the membrane.

[0016] An exemplary porous ("open-pore") filter membrane can be in the form of a thin film or sheet-type membrane comprising two opposing sides (i.e., two opposing surfaces) and a thickness between the two sides. Between the two opposing sides, along the thickness of the membrane, there is an open-cell structure comprising a three-dimensional void microstructure in the form of open cells defined by the matrix of solid polymer material forming the porous filter membrane. These cells are interconnected, i.e., "open cells" that allow liquid fluid to pass through the thickness of the membrane from one side of the membrane to the opposite side of the membrane. The open cells can be referred to as openings, pores, channels, or passages, and are mostly interconnected between adjacent cells to allow liquid fluid to pass through the thickness of the membrane.

[0017] In the described membranes, the open pore structure is distributed throughout the membrane thickness, with pores arranged with different pore sizes and different average pore sizes present in different portions of the membrane, i.e., different regions of the membrane thickness. The membrane includes a first side (sometimes referred to as the "dense" or "retention" side) that includes a distribution of relatively smaller pores and a second side (the "open" or "support" side) that includes a distribution of relatively larger pores. The dense side of the membrane has smaller pores on average, has higher retention properties, and can exhibit higher resistance to flow through the membrane due to its smaller pores (on average). The dense side exhibits higher resistance to flow compared to the open side, and inhibits the flow of liquid through the filter to a greater extent than the open side. The open side has relatively larger pores and lower retention properties, reducing resistance to flow (compared to the dense side) and allowing a relatively higher flow rate of liquid through that portion of the filter.

[0018] Each of the "dense face" and "open face" is considered to refer to a portion of a film that includes one surface of the film together with a three-dimensional portion of the film that extends below the surface through the thickness of the film to a depth (or "thickness") below the surface. Thus, each of the "dense face" and "open face" is considered to include one surface of the film that is bounded by a three-dimensional portion of the film that resides below the surface, which may be characterized as having a thickness relative to the total thickness of the film, and which may further be characterized as having a width and length shared with the entire film.

[0019] Because the boundary between the dense and open faces of a membrane and between the polymer materials used to manufacture each face at interior locations within the membrane can be difficult to distinguish, the thickness of the dense and open faces may not always be identifiable by physical inspection of the membrane. The thickness of the dense or open face of a membrane and the relative magnitude of each thickness can instead be assessed by the relative amount (by mass or volume) of polymer or polymer solution used to form the dense face compared to the open face, or both, based on the characteristics of the coextrusion process used to manufacture the membrane. For example, the relative thickness of the dense and open faces can be measured as the relative flow rate (by volume or mass) of the polymer solution used to form the dense face relative to the flow rate of the polymer solution used to form the open face. As another example, the relative thickness of the dense and open faces can be measured as the amount (by weight) of polymer that is part of the extruded dense face polymer solution relative to the amount (by weight) of polymer in the extruded open face polymer solution.

[0020] In any event, the exemplary membranes described are considered to have a dense side having a thickness (relative to the total thickness of the membrane) that is less than the thickness of the open side, for example, based on the fact that the dense side is formulated to contain a smaller amount of polymer (by mass or volume) compared to the amount of polymer in the open side. Exemplary thicknesses of the dense and open sides of the membrane can be such that the dense side is 20-45% of the membrane's thickness and the open side is 55-80% of the membrane's thickness relative to the total combined thickness of the open and dense sides; for example, the membrane can include a dense side that is 25-40% of the membrane's thickness and an open / closed side that is 60-75% of the membrane's thickness relative to the total thickness of the open and dense sides.

[0021] The dense surface of the membrane functions as the membrane's retention portion and is responsible for the physical retention (capture) and removal of particles or impurities from the liquid fluid as the fluid passes through the membrane's pores. The dense surface can effectively function as the membrane's retention portion without being unnecessarily or excessively thick; in fact, a less thick (i.e., thinner) dense surface can be advantageous because it introduces a relatively low resistance to the flow of liquid through the membrane. Thus, such porous membranes can be fabricated to include a dense surface that is relatively thin (having a smaller thickness) compared to the thickness of the open surface of the membrane.

[0022] The open side of the membrane acts as a support for the retaining side and desirably provides less restriction to the flow of liquid through the membrane. The average size of the pores in the open side is larger than the average size of the pores in the dense side.

[0023] The membrane, including both the dense and open sides, can be made of a polymer comprising, consisting of, or consisting essentially of polyethylene comprising a single polyethylene composition (e.g., based on molecular weight) or a blend of two or more different polyethylene compositions (e.g., a blend of two or more polyethylene compositions having different molecular weights).

[0024] The term "polyethylene" refers to a polymer having, in part or substantially, a linear molecular structure of repeating -CH-CH- units. Polyethylene is a semicrystalline polymer that typically elongates before breaking, improving its toughness. Polyethylene can be made by reacting a monomer composition containing monomers comprising, consisting of, or consisting essentially of ethylene monomer. Thus, a polyethylene polymer can be a polyethylene homopolymer prepared by reacting a monomer consisting of, or consisting essentially of, ethylene monomer. Alternatively, a polyethylene polymer can be a polyethylene copolymer prepared by reacting a combination of ethylene and non-ethylene monomers comprising, consisting of, or consisting essentially of ethylene monomer in combination with another type of monomer, e.g., another α-olefin monomer, e.g., butene, hexene, or octane, or a combination thereof. In polyethylene copolymers, the amount of ethylene monomer used to produce the copolymer can be any useful amount relative to the non-ethylene monomer, e.g., at least 50, 60, 70, 80, or 90% (by weight) of ethylene monomer per total weight of all monomers (ethylene and non-ethylene) in the monomer composition used to prepare the ethylene copolymer.

[0025] As used herein, a composition (e.g., a monomer composition) described as "consisting essentially of" a component or specific combination of components is a composition that contains that component or specific combination of components and small or insignificant amounts of the following other materials, e.g., 3, 2, 1, 0.5, 0.1, or 0.05 weight percent or less of any other component or combination of components. A monomer composition described as containing a monomer "consisting essentially of" ethylene monomer is a monomer composition that contains ethylene monomer and small or insignificant amounts or less of other monomeric materials, e.g., 3, 2, 1, 0.5, 0.1, or 0.05 weight percent or less of any other monomer.

[0026] The filter membranes as described are made of a polymer that includes (e.g., comprises, consists essentially of, or consists of) polyethylene, a polymer commonly used in porous filter membranes. Polyethylene polymer compositions (components) vary in properties such as molecular weight, density, molecular weight distribution, and melt index. Polyethylenes having molecular weights substantially greater than 1,000,000 daltons are sometimes referred to as ultra-high molecular weight polyethylene (UPE). In the membranes herein, polyethylene components comprising polyethylene having average molecular weights greater than 500,000 daltons, e.g., greater than 1,000,000 daltons, e.g., in the range of 500,000 to 2,000,000 or 3,000,000 daltons, may be useful for the dense or open face of the membrane. Molecular weights of polymers, reported in "daltons," can be measured using known gel permeation chromatography (GPC) (also known as size exclusion chromatography (SEC)) techniques and equipment.

[0027] The filter membrane, e.g., the dense side of the filter membrane, the open side of the filter membrane, or both, can be made from a single polyethylene polymer component (having a particular average molecular weight and molecular weight range) or can be made from a blend of two or more different polyethylene polymer components (each component having a different average molecular weight and molecular weight range).

[0028] In some examples, the membrane or its dense or open surface comprises polyethylene provided by one or more polyethylene polymer components, and the membrane (or its surface) comprises, consists of, or consists essentially of at least 50, 60, 70, 80, or 90 wt. % polyethylene having an average molecular weight in the range of 500,000 to 3,000,000 daltons, e.g., 500,000 to 1,000,000 daltons, 1,500,000 daltons, or 2,000,000 daltons.

[0029] 1 is a schematic diagram of the described membrane. Membrane 100 includes (e.g., includes, consists essentially of, or consists of) a dense side 102 and an open side 112 and has a total thickness 120. Dense side 102 includes a dense side surface 104 and a dense side thickness 106. Open side 112 includes an open side surface 114 and an open side thickness 116. Dotted line 108 indicates the boundary between dense side 102 and open side 112, which is the approximate or theoretical location of the interface or split between the dense and open sides.

[0030] As shown, the thickness 106 of the dense side 102 is less than the thickness 116 of the open side 112. The difference in thickness is a result of the process characteristics of preparing the membrane 100 by co-extruding polymer compositions to produce the described dense and open side membranes 100 with different thicknesses and different morphologies (average pore sizes). The boundary 108 is approximate, and a clear boundary 108 is not necessarily discernible upon physical inspection of the membrane 100.

[0031] The described membranes can be characterized by features including (in addition to having the described open and dense sides) thickness (total thickness of the membrane), porosity, bubble point in one or two directions through the membrane, flow time, and retention.

[0032] The described porous membranes may be in the form of a sheet having a substantially uniform thickness across the width and length of the sheet, with thicknesses ranging from 30, 50 or 80 up to 200 microns, for example, in the range of 50 to 150 microns.

[0033] The described membranes can have a porosity that allows the membrane to be effective as described herein and allows a suitable flow rate of liquid to pass through the membrane while also removing an effective amount of contaminants or impurities from the liquid. Examples of useful membranes can have a porosity of up to 80%, for example, a porosity in the range of 60-80 percent, e.g., 60-70 percent or 40-60 percent. As used herein, and in the porous body art, the "porosity" (sometimes referred to as "void fraction") of a porous body is a measure of the void (i.e., "empty") space within the body as a percentage of the body's total volume, calculated as the ratio of the body's void volume to the body's total volume. A body with 0 percent porosity is completely solid.

[0034] The size of the pores ("pore size") in the membrane (i.e., the average size of the pores throughout the membrane or in different portions of the membrane) and the distribution of pores of different sizes in the membrane, in combination with the porosity and thickness of the membrane, provide the desired flow of liquid fluid through the membrane while also achieving a desired high level of filtration (e.g., as measured by retention).

[0035] The pore size of the membrane varies in different portions of the membrane, with the pores in the dense side being smaller than those in the open side. The pores in the dense side can be of an average size to provide a combination of useful filtration properties (as measured by retention) and desirable flow characteristics. Exemplary pore sizes in the dense side of the membrane can be about 10, 20, 30, or nanometers, or 0.05 microns, up to about 10 microns, for example, within a range of sizes that may be classified as "microporous," "ultraporous," or "nanoporous." For purposes of this specification and claims, the term "microporous" may be used to refer to pores within any of these size ranges, including microporous and submicroporous sizes, as a way to distinguish from materials with larger pore sizes, i.e., materials that are considered "macroporous." Exemplary average pore sizes in the open side of the described membranes can be within these same ranges, but larger than the pores in the dense side.

[0036] The pore size of a membrane need not necessarily be measured directly, but can be assessed based on correlation with a property known as the "bubble point" (herein meaning "average bubble point"), which is an understood property of porous filter membranes. The bubble point corresponds to the pore size, which in turn can correspond to filtration performance, measured, for example, by retention. Smaller pore sizes can correlate with higher bubble points, and often with better filtration performance (higher retention). However, higher bubble points also typically correlate with relatively higher resistance to flow through the porous material and higher flow times (higher resistance to flow and lower flow rates for a given pressure drop). Exemplary filter membranes herein can exhibit a combination of relatively high bubble points, good filtration performance, and useful levels of flow, e.g., flow rates or "flow times," that allow the filter membranes to be used in commercial filtration processes.

[0037] For purposes of this disclosure, the mean bubble point is determined using the following procedure, hereafter referred to as the "mean bubble point test." A dry sample of membrane is placed in a holder and gas pressure is gradually applied to the dense side of the dry membrane using compressed air. The air flow rate through the dry membrane is measured as a function of pressure. The membrane is then wetted with ethoxy-nonafluorobutane HFE-7200 (available from 3M). Gas pressure is gradually applied to the dense side of the wet membrane using compressed air. The air flow rate through the wet membrane is measured as a function of pressure. This test is performed at ambient temperature (e.g., about 25 degrees Celsius, but not temperature controlled). The mean bubble point is the pressure at which the ratio of air flow through the wet membrane to the air flow through the dry membrane is 0.5.

[0038] Examples of useful average bubble points of the described porous filter membranes, measured using the Average Bubble Point Test, can be at least 50, 80, 90, 100, or 120 pounds per square inch (psi) or more, for example, up to 200 or 300 pounds per square inch, while the membranes also exhibit the useful properties of flow time and retention described elsewhere herein.

[0039] The described membranes can exhibit a useful, effective level of resistance to the flow of liquid through the membrane, combined with a desired bubble point and filtration performance. The resistance to the flow of liquid through the membrane can be measured in terms of flow rate or flow time (which is the reciprocal of flow rate). The described membranes can preferably have a useful or relatively low flow time, preferably combined with a relatively high bubble point and good filtration performance. The level of effectiveness of a filter membrane in removing undesirable materials (i.e., "impurities") from a liquid can be measured in one way as "retention." Retention, in relation to the effectiveness of a filter membrane (e.g., the described filter membranes), generally refers to the total amount of impurities removed from a liquid containing impurities relative to the total amount of impurities present in the liquid before the liquid passes through the filter membrane. Thus, the "retention" value of a filter membrane is a percentage, and a filter with a high retention value (higher percentage) is relatively more effective at removing particles from a liquid, while a filter with a low retention value (lower percentage) is relatively less effective at removing particles from a liquid. Membranes prepared according to the exemplary methods herein may exhibit filtration performance, as measured by retention, at least equivalent to commercially available filter membranes prepared from comparable materials (e.g., polyethylene) having comparable, nearly comparable, or somewhat similar thicknesses, as well as flow characteristics and bubble points (measured in flow time) within broadly similar ranges. As shown in the examples below, the membranes described herein have lower flow times relative to bubble points when compared to previous membranes; in other words, previous comparative membranes do not share both the bubble point and flow time characteristics of membranes having the properties disclosed herein.

[0040] In certain examples, the membranes herein can exhibit a useful or improved combination of bubble point (average bubble point) and flow characteristics (e.g., as measured by flow time) of the liquid passing through the membrane. Compared to comparable polyethylene porous filter membranes, useful or preferred membranes herein can have a highly desirable combination of increased bubble point over similar flow times. Exemplary membranes can exhibit higher bubble points over equal flow times across a range of bubble points relative to flow times, or alternatively, reduced (improved) flow times at the same bubble points. Exemplary membranes can exhibit flow time and bubble point characteristics such as: flow times of less than 2000 seconds and average bubble points of 75 psi or greater; flow times of less than 3000 seconds and average bubble points of 100 psi or greater; flow times of less than 4000 seconds and average bubble points of 125 psi or greater; or flow times of less than 6000 seconds and average bubble points of 150 psi or greater; or flow times of less than 10000 seconds and average bubble points of 175 psi or greater. These membranes also exhibit useful levels of filtration as measured by "retention," eg, filtration performance within the same range as other polyethylene filters of comparable thickness.

[0041] For purposes of this disclosure, flow time is determined using the following procedure, hereafter referred to as the "Flow Time Test." To measure flow time, isopropyl alcohol (IPA) is applied to the open face (larger pore size) of a 47 mm membrane disk at a pressure of 14.2 psi. If the pressure is different from 14.2 psi, the flow time is normalized to 14.2 psi. The time required to flow a certain amount of fluid through the membrane is measured, and the time required to flow 500 mL is calculated. The temperature of the fluid is also measured, and the time is corrected for changes in viscosity with temperature using the following equation, normalized to 21°C: Flow time (s) = Measurement time (s) * [500 (ml) / Measurement volume (ml)] * [Measurement pressure (psi) / 14.2 (psi)] * Viscosity correction Viscosity correction = measurement temperature (℃)*0.0313+0.356

[0042] By another preferred means, the exemplary membranes described can exhibit flow time and bubble point properties such as: flow times of less than 1500 seconds and an average bubble point of 75 psi or greater; flow times of less than 2500 seconds at an average bubble point of 100 psi or greater; flow times of less than 3000 seconds at an average bubble point of 125 psi or greater; flow times of less than 5000 seconds at an average bubble point of 150 psi or greater; and flow times of less than 8000 seconds at an average bubble point of 175 psi or greater. These membranes also exhibit useful levels of filtration, measured by "retention," e.g., filtration performance within the range of other polyethylene filters of comparable thickness.

[0043] Stated to characterize the range of maximum flow times of a filter relative to its mean bubble point, the described exemplary membranes can exhibit a log 10 flow time (seconds) relative to a measured mean bubble point (lbs / in 2 ) that is less than the log 10 flow time relative to the mean bubble point, according to the formula: log 10 (flow time) = 2.757 + 0.007105 * (mean bubble point). In other embodiments, the described exemplary membranes can exhibit a log 10 flow time (seconds) relative to a measured mean bubble point (lbs / in 2 ) that is less than or equal to the log 10 flow time relative to the mean bubble point, according to the formula: log 10 (flow time) = 2.707 + 0.006485 * (mean bubble point). In some embodiments, the described exemplary membranes can exhibit a log flow time (seconds) for a measured average bubble point (pounds per square inch) that is less than the log flow time for the average bubble point, according to the formula: log(flow time) = 2.757 + 0.007105 * (average bubble point), and is equal to or greater than the log flow time for the average bubble point (pounds per square inch), according to the formula: log(flow time) = 2.4888 + 0.006593 * (average bubble point). In some embodiments, the described exemplary membranes can exhibit a log flow time (seconds) for a measured average bubble point (pounds per square inch) that is less than the log flow time for the average bubble point, according to the formula: log(flow time) = 2.707 + 0.006485 * (average bubble point), and is equal to or greater than the log flow time for the average bubble point (pounds per square inch). In some embodiments, the exemplary membranes described can exhibit a log flow time (seconds) for a measured mean bubble point (pounds per square inch) that is 5% or 10% shorter than the flow time for the mean bubble point, according to the formula log flow time = 2.757 + 0.007105 * (mean bubble point) or the formula log flow time = 2.707 + 0.006485 * (mean bubble point).

[0044] The process for making the described porous filter membranes can be an "extrusion melt-cast" process, a type of method sometimes referred to as "thermally induced liquid-liquid phase separation," carried out by co-extruding two polymer streams (two different heated polymer solutions) to form the described membranes, which contain a dense side and an open side.

[0045] In this type of process, a polymer (e.g., polyethylene) is generally dissolved in one or more solvents at an elevated temperature (the "extrusion temperature") to form a heated polymer solution that can be processed and shaped, for example, by an extruder. The heated polymer solution is passed through an extruder and extrusion die, where it can solidify into a desired shape, such as in the form of a sheet membrane. The heated polymer solution passes through the die and is dispensed onto a forming surface that is at a temperature much lower than the extrusion temperature, i.e., the "cooling temperature." When the extruded heated polymer solution contacts the cold forming surface, the polymer and solvent of the heated polymer solution undergo one or more phase separations such that the polymer is formed into an open-pore porous membrane.

[0046] A heated polymer solution can be prepared containing polyethylene (as described herein) dissolved in a solvent comprising a first ("strong") solvent and a second ("weak") solvent. The polymer of the polymer solution can comprise, consist of, or consist essentially of polyethylene as described herein.

[0047] A strong solvent can substantially dissolve the polymer in the heated polymer solution. Examples of useful strong solvents include organic liquids in which the polyethylene polymers described herein are highly soluble at extrusion temperatures and in which the polyethylene polymers have low solubility at cooling temperatures. Examples of useful strong solvents include mineral oil and kerosene.

[0048] A weak solvent is one in which the polyethylene polymer has low solubility at the extrusion temperature and the cooling temperature, is miscible with the strong solvent at the extrusion temperature, and is immiscible with the strong solvent at the cooling temperature. Specific examples of weak solvents include dioctyl phthalate, dibutyl sebacate (DBS), dioctyl sebacate, di(2-ethylhexyl) phthalate, di(2-ethylhexyl) adipate, dibutyl phthalate, tetralin, n-decanol, 1-dodecanol, diphenylmethane, and mixtures thereof.

[0049] The amount of polymer (e.g., polyethylene or polyethylene with one or more other polymers) contained in the heated polymer solution can be high enough relative to the amount of solvent so that the heated polymer solution can be processed by extrusion through an extruder and die, and low enough so that the polymers in the polymer solution can coalesce to form the desired porous morphology upon casting and cooling. Useful or preferred amounts of the polymers described herein that can be included in the described heated polymer solutions and processed as described can range from 5, 10, or 15 up to 35 weight percent, e.g., 17 to 20, 25, or 30 weight percent, of polymer based on the total weight of the heated polymer solution. The remainder of the heated polymer solution can be a combination of one or more weak solvents and one or more strong solvents. Thus, useful or preferred heated polymer solutions can contain, for example, 65 to 85, 90, or 95 weight percent solvent (a combination of weak and strong solvents), e.g., 70 to 75, 80, or 83 weight percent solvent based on the total weight of the heated polymer solution.

[0050] The relative amount of strong solvent to weak solvent can be selected as desired to achieve the desired pore structure of the porous membrane. A higher relative amount of strong solvent can produce a filter membrane with smaller pores. A higher relative amount of weak solvent can produce a filter membrane with larger pores. Useful relative amounts of strong solvent to weak solvent can vary within ranges including 10:90 to 90:10, 20:80 to 80:20, 25:75 to 75:25, and 40:60 to 60:40 (strong solvent:weak solvent).

[0051] When the heated polymer solution is rapidly cooled, several physical changes in the polymer solution cause the extruded heated polymer solution to form a porous filter membrane. One change is that the quenching of the heated polymer solution causes the solution to phase separate into two liquid phases: a strong solvent liquid phase containing a high level of dissolved polymer and a weak solvent liquid phase containing a low amount of dissolved polymer. A further change caused by quenching is that the polymer dissolved in the strong solvent coalesces and precipitates from the strong solvent as a solid polymer phase.

[0052] Useful processes can be based, more particularly, on thermally induced phase separation processes involving liquid-liquid phase separation of a weak solvent and a strong solvent (including a dissolved polymer). According to such methods, a heated polymer solution containing a polymer (including, consisting of, or consisting essentially of the described polyethylene) dissolved in a strong solvent, further combined with a second solvent (also called a "weak solvent" or "non-solvent" or "porogen"), forms a heated polymer solution. This heated polymer solution system is characterized by having a temperature range over which the solution maintains a homogeneous solution of the polymer dissolved in the combination of strong and weak solvents, and a second (lower) temperature range over which the solution phase separates.

[0053] By cooling the heated polymer solution from a high temperature ("extrusion") to a low temperature ("cooling"), the heated polymer solution initially separates into two liquid phases: a strong solvent phase with a high dissolved polymer content and a weak solvent phase with a low dissolved polymer content. Upon further cooling below the solidification temperature, the high polymer content phase solidifies to form a three-dimensional membrane structure. The rate at which the heated polymer solution is cooled can affect the pore structure that is produced. Generally, faster cooling results in the formation of smaller pores.

[0054] The heated polymer solution formed from the polymer, weak solvent, and strong solvent can be extruded during a heated extrusion process and passed through an extrusion die to be shaped as desired. Many examples of useful extrusion equipment are known and commercially available, one commercially available example being a Leistritz 27 mm twin-screw co-rotating extruder. Conventional dies such as sheeting dies, casting molds, doctor blades, profiled dies, and the like are also well known and will be understood to be useful in accordance with the present invention.

[0055] The extruded heated polymer solution can be cooled by contacting it with any forming surface, such as a chill roll or "chill roll."

[0056] Useful or preferred extrusion temperatures, ie the temperature of the heated polymer solution exiting the extrusion die, may range, for example, from 180°C to 250°C, such as from 195°C to 220°C.

[0057] Useful or preferred cooling temperatures, for example the temperature of the surface onto which the heated polymer solution is extruded, such as a surface chill roll, can range, for example, from 10°C to 50°C, such as from 25°C to 40°C.

[0058] According to this specification, porous membranes can be formed by an "extrusion melt casting" process (including "thermally induced liquid-liquid phase separation") using a co-extrusion method involving the flow and extrusion of two heated polymer solutions. One heated polymer solution, referred to as the dense-face heated polymer solution, is formed and extruded using a co-extrusion method to form the dense side of the membrane. The second heated polymer solution, referred to as the open-face heated polymer solution, is formed and extruded using a co-extrusion method to form the open side of the membrane.

[0059] According to the methods of the present invention, the characteristics of the co-extrusion process and the characteristics of the two different heated polymer solutions can be selected and controlled to produce the described porous filter membranes having dense and open faces with the described morphologies and relative thicknesses, and having the described flow and bubble point properties, along with effective filter retention characteristics.

[0060] Various characteristics of the coextrusion process can be selected and controlled to produce the described membranes having the described dense and open sides, with the open side having larger pores and greater thickness compared to the dense side. These include the composition of the first heated polymer solution and its polymer (polyethylene); the composition of the second heated polymer solution and its polymer (polyethylene); and the relative amounts (relative flow rates in mass / hour, e.g., pounds / hour) of the first and second heated polymer solutions flowing through the extruder to form the coextruded membrane, which can be controlled by the thickness of each extruded layer, as can be affected by the respective flow rates through the extrusion die.

[0061] The membranes produced have a dense side that has a thickness (relative to the total thickness of the membrane) that is less than the thickness of the open side and that is a smaller portion of the total thickness of the membrane compared to the thickness of the open side. Exemplary membranes that are considered to have a dense side that has a thickness that is less than the thickness of the open side of the membrane can have a dense side that contains less polymer than the amount of polymer contained in the open side. The dense side of the exemplary membranes described can contain 15-40 wt. % of the total amount of polymer of the dense side and the open side, e.g., 25-35 wt. % of the total amount of polymer of the dense side and the open side of the membrane. The open side of the exemplary membrane will contain 60-80 wt. % of the total amount of polymer of the dense side and the open side, e.g., 65-75 wt. % of the total amount of polymer of the dense side and the open side of the membrane.

[0062] The amount of polymer comprising the dense side relative to the amount of polymer comprising the open side can be influenced or controlled by characteristics of the coextrusion process, such as the relative flow rates of the dense side heating polymer solution and the open side heating polymer solution. In an exemplary process, the dense side heating polymer solution has a lower flow rate (e.g., mass / hour) from the die during the coextrusion process than the flow rate of the open side heating polymer solution. As a specific example, the flow rate of the dense side heating polymer solution can be in the range of 15 to 40 weight percent of the total (combined) flow rate of the dense side heating polymer solution and the open side heating polymer solution from the coextrusion die, e.g., the flow rate of the dense side heating polymer solution can be in the range of 25 to 35 weight percent of the total flow rate (by mass) of both the dense side heating polymer solution and the open side heating polymer solution. The flow rate of the open-side heated polymer solution can be in the range of 60 to 80 weight percent of the total (combined) flow rate (by mass) of the dense-side heated polymer solution and the open-side heated polymer solution from the coextrusion die, for example, the flow rate of the open-side heated polymer solution can be in the range of 65 to 75 weight percent of the total flow rate of both the dense-side heated polymer solution and the open-side heated polymer solution.

[0063] Additionally or optionally, the dense side heated polymer solution may contain a higher concentration of polymer (by weight) relative to the concentration of polymer in the open side heated polymer solution to affect the morphology (e.g., average pore size) of the dense side compared to the open side of the membrane. A higher concentration of polymer in the heated polymer solution may result in relatively smaller pores in the coagulated film upon solidification compared to pores formed from a heated polymer solution containing a lower concentration of polymer.

[0064] As specific examples, an exemplary dense surface heating polymer solution can contain 10 to 30 weight percent, e.g., 12 to 25 weight percent, of polymer. An exemplary open surface heating polymer solution can contain 5 to 20 weight percent, e.g., 8 to 15 weight percent, of polymer.

[0065] 2A, a side view of a coextrusion system useful for making a porous filter membrane 220 as described herein by a coextrusion method is shown. The coextrusion system 200 includes an extruder 202 for extruding a stream of a first heated polymer solution (dense side heated polymer solution) 208 and an extruder 204 for extruding a stream of a second heated polymer solution (open side heated polymer solution) 206. During operation, the dense side heated polymer solution 208 increases in density (PC TS ) (mass of polymer / volume of polymer solution or mass of polymer solution), and the extruder 202 and die 212 are controlled to have a flow rate (F TS ) (mass or volume of polymer solution / time). The dense side heated polymer solution passes through die 212 and is positioned to contact chill roll 210 as dense side 224 of membrane 220. The open side heated polymer solution 206 flows through the die 212 at a rate of 1000 rpm. The dense side 224 of membrane 220 is positioned to contact chill roll 210. The open side heated polymer solution 206 flows through the die 212 at a rate of 1000 rpm. The dense side 224 of membrane 220 is positioned to contact chill roll 210 as dense side 224 of membrane 2 ... OS ) (mass of polymer / volume of polymer solution or mass of polymer solution), and the extruder 204 and die 214 are controlled to have a flow rate (F OS ) (mass or volume of polymer solution / time). Open-face heated polymer solution 206 passes through die 214 and is deposited as open face 222 of membrane 220 onto dense face 224.

[0066] When the two streams of heated polymer solution 206 and 208 are formed as layers 222 and 224 on the chilled surface of chill roll 210, phase separation and solidification of the polymers present in the heated polymer solution occurs, forming a porous membrane with the described dense and open sides. Dense side 224 solidifies rapidly by intimate contact with the surface of chill roll 221. The rapid solidification results in the formation of smaller pores compared to the pores formed in open side 222, which form more slowly due to the lack of direct contact with chill roll 210.

[0067] 2B, there is shown a schematic side view of an alternative coextrusion system useful for making the porous filter membranes 320 described herein by a coextrusion method using a single die 312. The coextrusion system 300 includes an extruder 302 for extruding a stream of a first heated polymer solution (dense side heated polymer solution) 308 and an extruder 304 for extruding a stream of a second heated polymer solution (open side heated polymer solution) 306. During operation, the dense side heated polymer solution 308 increases in density (PC TS ) (mass of polymer / volume of polymer solution or mass of polymer solution), and the extruder 302 and die 312 are controlled to have a flow rate (F TS ) (mass or volume of polymer solution / time). The dense side heated polymer solution passes through die 312 and die opening 314 and is positioned to contact chill roll 310 as dense side 324 of membrane 320. The open side heated polymer solution 306 flows through the die 312 at a rate of 1000 rpm. The dense side 324 of membrane 320 is positioned to contact chill roll 310. The open side heated polymer solution 306 flows through the die 312 at a rate of 1000 rpm. The dense side 324 of membrane 320 is positioned to contact chill roll 310 as ... OS ) (mass of polymer / volume of polymer solution or mass of polymer solution), and the extruder 304 and die 312 are controlled to have a flow rate (F OS ) (mass or volume of polymer solution / time). Open-face heated polymer solution 306 flows through die 312 and die opening 314 simultaneously with the flow of dense-face heated polymer solution 308, and emerges as open face 322 of membrane 320 adjacent to (above) dense face 324.

[0068] As the two streams of heated polymer solutions 306 and 308 form as layers 322 and 324 on the chilled surface of chill roll 310, phase separation and solidification of the polymers present in the heated polymer solutions occurs, forming a porous membrane with the described dense and open sides. Dense side 324 solidifies rapidly by intimate contact with the surface of chill roll 321. The rapid solidification results in the formation of smaller pores compared to the pores formed in open side 322, which form more slowly due to the lack of direct contact with chill roll 310.

[0069] The co-extrusion process parameters of system 200 or 300 can be selected and controlled to achieve the desired morphology of each of the dense and open faces, and the desired relative thicknesses of the dense and open faces. These parameters include the flow rates of the two heated polymer solutions, i.e., (F TS and F OS ), and the concentration of the polymer in each of the heated polymer solutions (PC TS and PC OS For example, the flow rate on the dense side can be lower than the flow rate on the open side to create a thickness on the open side that is greater than the thickness on the dense side (F TS <F OS ), examples of specific relative flow rates of the two heated polymer solutions are described elsewhere herein. Additionally or alternatively, the polymer concentration on the dense side can be higher than the polymer concentration on the open side to form smaller pores on the dense side compared to the open side (PC TS >PC OS ).

[0070] In commercial melt-casting processes for forming porous polymeric membranes, an optional step is to stretch the membrane after it has been extruded and solidified to form a solid membrane. The stretching step reduces the membrane's thickness by using a force to stretch the cast membrane in the length direction, width direction, or both, after extrusion and cooling. The shape of the open pores in the membrane is affected, for example, by the stretching in the direction of stretching.

[0071] In contrast to melt-casting processes, which involve stretching a melt-cast film in one or both directions, either length, width, or both, the porous films described herein do not require, and may eliminate, stretching steps in one direction (length or width) or both width and length. The described films do not require stretching of the film in the length or width direction to exhibit the described flow and bubble point. For example, the described films may be made without any stretching steps, or with only slight stretching between the melt-casting of the film and its installation in a filter product, such as a filter cartridge. The film may be processed without any stretching or with minimal stretching, by a process that does not cause the film to stretch (permanently deform) in one or both directions, for example, by more than 5, 2, or 1 percent.

[0072] The filter membranes described herein, or filters or filter components containing the filter membranes, can be useful in methods for filtering liquid chemical materials to purify or otherwise remove unwanted materials from the liquid chemical material to produce high-purity liquid chemical materials, particularly useful in industrial processes requiring the input of chemical materials with very high levels of purity. Generally, the liquid chemical may be any of a variety of useful commercial materials or liquid chemicals useful in any of a variety of industrial or commercial applications. Particular examples of the filter membranes described can be used to filter liquid solvents or other process solutions used in, for example, semiconductor photolithography methods, wet etching or cleaning processes, methods for forming spin-on-glass (SOG), back surface antireflective coating (BARC) methods, etc., to purify liquid chemicals used or useful in semiconductor or microelectronic manufacturing applications.

[0073] Some specific, non-limiting examples of liquid solvents that can be filtered using the described filter membranes include n-butyl acetate (nBA), isopropyl alcohol (IPA), 2-ethoxyethyl acetate (2EEA), cyclohexanone, ethyl lactate, gamma-butyrolactone, hexamethyldisilazane, methyl 2-hydroxyisobutyrate, methyl isobutylcarbinol (MIBC), n-butyl acetate, methyl isobutyl ketone (MIBK), isoamyl acetate, propylene glycol monoethyl ether, propylene glycol methyl ether (PGME), 2-heptanone, and propylene glycol monomethyl ether acetate (PGMEA).

[0074] The filter membrane can be contained within a larger filter structure, such as a filter or filter cartridge, used in a filtration system. The filtration system places the filter membrane in a liquid chemical flow path, for example, as part of a filter or filter cartridge, so that the filter membrane can remove impurities and contaminants from the liquid chemical. The filter or filter cartridge structure can include one or more various additional materials and structures that support the porous filter membrane within the filter and allow fluid to flow from the filter inlet through the filter membrane to the filter outlet, thereby passing through the filter membrane as it passes through the filter. The filter membrane supported by the filter structure can be any useful shape, such as a pleated cylinder, a cylindrical pad, one or more non-pleated (flat) cylindrical sheets, a pleated sheet, etc.

[0075] An example of a filter structure including a pleated cylindrically-formed filter membrane can be fabricated to include the following component parts, any of which may be included in the filter structure but may not be required: a rigid or semi-rigid core that supports the pleated cylindrically-coated filter membrane at its internal opening, a rigid or semi-rigid cage that supports or surrounds the outside of the pleated cylindrically-coated filter membrane on the outside of the filter membrane, optional end pieces or "packs" located at each of the two opposing ends of the pleated cylindrically-coated filter membrane, and a filter housing that includes an inlet and an outlet. The filter housing can be of any useful and desired size, shape, and material, and can preferably be made from a suitable polymeric material.

[0076] As an example, FIG. 3 shows a filter component 430 that is the product of a pleated cylindrical component 410 and an end piece 422, along with any other optional components. The cylindrical component 410 includes a filter membrane 412 and is pleated, as described herein. The end piece 422 is attached (e.g., "sealed") to one end of the cylindrical filter component 410. The end piece 422 may preferably be made of a melt-processable polymeric material. A core (not shown) may be disposed in the interior opening 424 of the pleated cylindrical component 410, and a cage (not shown) may be disposed around the exterior of the pleated cylindrical component 410. A second end piece (not shown) may be attached ("sealed") to the second end of the pleated cylindrical component 430. The resulting pleated cylindrical component 430, with its two opposing sealed ends and optional core and cage, may then be placed within a filter housing that includes an inlet and an outlet and is configured so that any fluid entering the inlet must pass through the filter membrane 412 before exiting the filter at the outlet.

[0077] The filter housing can be of any useful and desired size, shape and material, and preferably can be a fluorinated or non-fluorinated polymer, such as nylon, polyethylene, or a fluorinated polymer, such as poly(tetrafluoroethylene-co-perfluoro(alkyl vinyl ether)), TEFLON® perfluoroalkoxyalkane (PFA), perfluoromethylalkoxy (MFA), or another suitable fluoropolymer (e.g., perfluoropolymer). [Example]

[0078] Referring to FIG. 4, a scatter plot of log flow time (seconds) versus mean bubble point (psi) is shown for three filter membranes: Filter Membrane 1 ("High Flow"—circles), Filter Membrane 2 ("Extra High Flow"—triangles), and a comparative (not of the present invention) filter membrane (top points marked with an "x"). Filter Membrane 1 was made from a single polymer and had an average molecular weight of about 1.70 M Daltons and a thickness of about 80 microns. Filter Membrane 2 was made from a blend of two polymers and had an average molecular weight of about 1.15 M Daltons and a thickness of about 100 microns. The comparative filter membrane was made from a blend of two polymers and had an average molecular weight of about 2.60 M Daltons and a thickness of about 50 microns. The mean bubble points were determined using the Mean Bubble Point Test described above, and the flow times were determined using the Flow Time Test described above.

[0079] As shown, filter membranes 1 and 2 exhibit very favorable flow characteristics, as indicated by the shortened flow times at higher mean bubble points. At a mean bubble point of approximately 150 psi, filter membrane 1 has a flow time of approximately 6000 seconds or less, filter membrane 2 has a flow time of approximately 4000 seconds or less, while the comparative filter membrane has a flow time of over 9000 seconds. Also, as shown in Figure 4, a boundary exists between the comparative filter membrane and filter membranes 1 and 2. The comparative filter membrane has a log(flow time) greater than 2.757 + 0.007105 * (mean bubble point). Filter membranes 1 and 2 have a log(flow time) less than 2.757 + 0.007105 * (mean bubble point) and a log(flow time) greater than or equal to 2.4888 + 0.006593 * (mean bubble point). Filter membrane 1 typically has a log10(flow time) less than 2.757 + 0.007105 * (mean bubble point) and a log10(flow time) greater than 2.707 + 0.006485 * (mean bubble point). Filter membrane 2 typically has a log10(flow time) equal to or less than 2.707 + 0.006485 * (mean bubble point) and a log10(flow time) equal to or greater than 2.4888 + 0.006593 * (mean bubble point). Aspects of the Disclosure

[0080] In a first aspect of the present disclosure, a porous polyethylene membrane comprising a first surface and an opposing second surface, and a thickness between the first surface and the second surface, wherein the membrane exhibits a log flow time (seconds) to mean bubble point (pounds per square inch) that is less than the log flow time (seconds) to mean bubble point according to the formula: log(flow time) = 2.757 + 0.007105 * (mean bubble point), wherein the flow time is measured using a flow time test and the mean bubble point is measured using a mean bubble point test.

[0081] In a second aspect of the first aspect, the membrane exhibits a log flow time (seconds) to mean bubble point (pounds per square inch) that is less than or equal to the log flow time to mean bubble point according to the formula: log(flow time)=2.707+0.006485*(mean bubble point).

[0082] In a third aspect related to the first or second aspects, the first surface comprises polyethylene having a first average molecular weight and the second surface comprises polyethylene having a second average molecular weight, the first molecular weight equal to the second molecular weight.

[0083] In a fourth embodiment of any preceding embodiment, the membrane has a thickness in the range of 30 to 200 microns.

[0084] In a fifth aspect according to the first aspect, the membrane exhibits a log flow time (seconds) to mean bubble point (pounds per square inch) that is 5% below the log flow time to mean bubble point according to the formula: log(flow time)=2.757+0.007105*(mean bubble point).

[0085] In a sixth embodiment according to the second embodiment, the membrane exhibits a log flow time (seconds) to mean bubble point (pounds per square inch) that is 5% less than the log flow time to mean bubble point according to the formula: log(flow time)=2.707+0.006485*(mean bubble point).

[0086] In a seventh embodiment according to any of the preceding embodiments, the membrane exhibits a log flow time (seconds) to mean bubble point (pounds per square inch) that is greater than or equal to the log flow time to mean bubble point according to the formula: log(flow time)=2.4888+0.006593*(mean bubble point).

[0087] In an eighth embodiment, a filter cartridge includes the membrane of any of the preceding embodiments, the filter cartridge including a filter housing including an inlet, an outlet, and a membrane supported within the housing between the inlet and the outlet such that liquid entering the inlet passes through the membrane before passing through the outlet.

[0088] In a ninth aspect, a method of using the filter cartridge of the eighth aspect includes passing a fluid through the inlet, through the membrane, and out the outlet, wherein the fluid is useful in a semiconductor manufacturing process.

[0089] In a tenth embodiment, a method for making a coextruded porous polyethylene membrane having a first side and two opposing sides, and a thickness between the first side and the second side, and having pores throughout the thickness, comprises co-extruding a first heated liquid polymer solution and a second heated liquid polymer solution, wherein the first polymer solution comprises polyethylene in a liquid solvent, and the second polymer solution comprises polyethylene in a liquid solvent; and reducing the temperature of the co-extruded liquid polymer solutions to solidify the polymers of the liquid polymer solutions to form a membrane, wherein the membrane comprises a dense side formed from the first polymer solution and an open side formed from the second polymer solution, wherein the membrane exhibits a log10 flow time (seconds) for the mean bubble point (pounds per square inch) that is less than the log10 flow time for the mean bubble point according to the formula: log10(flow time) = 2.757 + 0.007105 * (mean bubble point), wherein the flow time is measured using a flow time test and the mean bubble point is measured using an average bubble point test.

[0090] An eleventh aspect related to the tenth aspect, further comprising extruding the first polymer solution at a flow rate in the range of 15 to 40% of the total flow rate (mass / time) of the first polymer solution and the second polymer solution.

[0091] A twelfth aspect related to the tenth or eleventh aspect, further comprising extruding a first polymer solution having a first concentration of polymer in the first polymer solution and extruding a second polymer solution having a second concentration of polymer in the second polymer solution, wherein the first concentration is greater than the second concentration.

[0092] A thirteenth aspect according to any of the tenth to twelfth aspects, further comprising co-extruding the first heated polymer solution and the second heated polymer solution at an extrusion temperature, and reducing the temperature of the co-extruded heated polymer solutions by contacting the first heated polymer solution with a surface having a temperature below the extrusion temperature.

[0093] In a fourteenth aspect relating to any of the tenth to thirteenth aspects, the first heated polymer solution forms a dense layer of the membrane having pores with an average pore size, and the second heated polymer solution forms an open layer of the membrane having pores with an average pore size larger than the average pore size of the pores in the dense porous portion.

[0094] In a fifteenth aspect relating to any of the tenth to twelfth aspects, the membrane has a thickness in the range of 30 to 200 microns.

[0095] In a sixteenth aspect relating to any of the tenth to fifteenth aspects, the first aspect comprises polyethylene having an average molecular weight in the range of 500,000 daltons to 3,000,000 daltons, and the second aspect comprises polyethylene having an average molecular weight in the range of 500,000 daltons to 3,000,000 daltons.

[0096] In a seventeenth aspect relating to any of the tenth through sixteenth aspects, the first aspect comprises polyethylene having an average molecular weight in the range of 500,000 daltons to 2,000,000 daltons, and the second aspect comprises polyethylene having an average molecular weight in the range of 500,000 daltons to 2,000,000 daltons.

[0097] In an eighth aspect according to any of the tenth to twelfth aspects, the membrane exhibits a log flow time (seconds) to mean bubble point (pounds per square inch) that is less than or equal to the log flow time to mean bubble point according to the formula: log(flow time)=2.707+0.006485*(mean bubble point).

[0098] In a nineteenth aspect, a method of making a filter cartridge includes making a membrane according to the method of any of the tenth to eighteenth aspects, and placing the membrane in a filter housing including an inlet, an outlet, and the membrane supported within the housing between the inlet and the outlet such that liquid entering the inlet passes through the membrane before passing through the outlet.

[0099] In a twentieth aspect relating to the nineteenth aspect, the membrane is made by the described coextrusion method and is unstretched when placed in the filter housing.

[0100] Having thus described several exemplary embodiments of the present disclosure, those skilled in the art will readily appreciate that still other embodiments may be made and used within the scope of the appended claims. Many advantages of the present disclosure encompassed by this specification have been set forth in the foregoing description. It will be understood, however, that the present disclosure is in many respects merely illustrative. Changes may be made in detail without departing from the scope of the present disclosure. The scope of the present disclosure will, of course, be defined in the language in which the appended claims are expressed.

Claims

1. First aspect, an opposing second surface; and Thickness between the first and second surfaces A porous polyethylene membrane comprising:

10. The membrane of claim 1, wherein the membrane has the formula: log10(flow time) = 2.757 + 0.007105 * (average bubble point) and a log 10 flow time (seconds) to the mean bubble point (pounds per square inch) that is less than the log 10 flow time (seconds) to the mean bubble point according to The flow time is measured using a flow time test, The average bubble point is measured using the average bubble point test. Porous polyethylene membrane.

2. 10. The membrane of claim 1, wherein the membrane has the formula: log10(flow time) = 2.707 + 0.006485 * (average bubble point) 2. The membrane of claim 1, wherein the membrane exhibits a log 10 flow time (seconds) to mean bubble point (pounds per square inch) that is less than or equal to the log 10 flow time to mean bubble point according to

3. the first surface comprises polyethylene having a first average molecular weight; the second surface comprises polyethylene having a second average molecular weight; the first molecular weight is equal to the second molecular weight; 3. The membrane of claim 1 or 2.

4. The membrane of any of claims 1 to 3, wherein the membrane has a thickness in the range of 30 to 200 microns.

5. 10. The membrane of claim 1, wherein the membrane has the formula: log10(flow time) = 2.757 + 0.007105 * (average bubble point) 10. The membrane of claim 1, wherein the membrane exhibits a log 10 flow time (seconds) to mean bubble point (pounds per square inch) that is 5% less than the log 10 flow time to mean bubble point according to

6. 10. The membrane of claim 1, wherein the membrane has the formula: log10(flow time) = 2.707 + 0.006485 * (average bubble point) 3. The membrane of claim 2, which exhibits a log 10 flow time (seconds) to mean bubble point (pounds per square inch) that is 5% less than the log 10 flow time to mean bubble point according to

7. 10. The membrane of claim 1, wherein the membrane has the formula: log10(flow time) = 2.4888 + 0.006593 * (average bubble point) 7. The membrane of claim 1, wherein the membrane exhibits a log 10 flow time (seconds) to mean bubble point (pounds per square inch) that is equal to or greater than the log 10 flow time to mean bubble point according to

8. 8. A filter cartridge comprising the membrane of claims 1 to 7 comprising a filter housing comprising an inlet, an outlet, and a membrane supported within the filter housing between the inlet and the outlet such that liquid entering the inlet passes through the membrane before passing through the outlet.

9. 10. A method of using the filter cartridge of claim 8, comprising passing a fluid through the inlet, through the membrane, and out the outlet, wherein the fluid is useful in a semiconductor manufacturing process.

10. 1. A method for making a coextruded porous polyethylene membrane having a first surface and two opposing surfaces and a thickness between the first surface and the second surface, the membrane having pores throughout the thickness, the method comprising: co-extruding a first heated liquid polymer solution and a second heated liquid polymer solution, the first polymer solution comprising polyethylene in a liquid solvent and the second polymer solution comprising polyethylene in a liquid solvent; reducing the temperature of the co-extruded liquid polymer solution to solidify the polymers of the liquid polymer solution to form a membrane, the membrane comprising a dense surface formed from the first polymer solution and an open surface formed from the second polymer solution; 1. The membrane has the formula: log10(flow time) = 2.757 + 0.007105 * (average bubble point) wherein the flow time is measured using a flow time test and the average bubble point is measured using an average bubble point test.

11. 11. The method of claim 10, further comprising extruding the first polymer solution at a flow rate in the range of 15 to 40% of the total flow rate (mass / time) of the first polymer solution and the second polymer solution.

12. extruding a first polymer solution having a first concentration of polymer in the first polymer solution; and extruding a second polymer solution having a second concentration of polymer in the second polymer solution; 12. The method of claim 10 or 11, wherein the first concentration is greater than the second concentration.

13. co-extruding the first heated polymer solution and the second heated polymer solution at an extrusion temperature; reducing the temperature of the co-extruded heated polymer solution by contacting the first heated polymer solution with a surface having a temperature below the extrusion temperature; 13. The method of any of claims 10 to 12, further comprising:

14. the first heated polymer solution forms a dense layer of the membrane having pores with an average pore size; the second heated polymer solution forms an open layer of the membrane having pores with an average pore size larger than the average pore size of the pores of the dense porous portion.

14. The method according to any one of claims 10 to 13.

15. A method according to any one of claims 10 to 14, wherein the membrane has a thickness in the range of 30 to 200 microns.

16. the first surface comprises polyethylene having an average molecular weight in the range of 500,000 daltons to 3,000,000 daltons; the second surface comprises polyethylene having an average molecular weight in the range of 500,000 daltons to 3,000,000 daltons; 16. The method according to any one of claims 10 to 15.

17. the first surface comprises polyethylene having an average molecular weight in the range of 500,000 daltons to 2,000,000 daltons; the second surface comprises polyethylene having an average molecular weight in the range of 500,000 daltons to 2,000,000 daltons; 17. The method according to any one of claims 10 to 16.

18. 1. The membrane of claim 1, wherein the membrane has the formula: log10(flow time) = 2.707 + 0.006485 * (average bubble point) 18. The method of claim 10, wherein the average bubble point (lbs / in 2 ) has a log 10 flow time (sec) that is equal to or less than the log 10 flow time (sec) for the average bubble point according to

19. 1. A method of making a filter cartridge, comprising: Producing a membrane according to the method of any one of claims 10 to 18; placing the membrane within a filter housing including an inlet, an outlet, and a membrane supported within the filter housing between the inlet and the outlet such that liquid entering the inlet passes through the membrane before passing through the outlet; A method comprising:

20. 20. The method of claim 19, wherein the membrane is made by the described coextrusion method and is unstretched when placed within the filter housing.

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